Error: "Transport reported error: 1255: A network-related or instance-specific error occured while establishing a connection to SQL server." (4263882)

Error: "Transport reported error: 1255: A network-related or instance-specific error occured while establishing a connection to SQL server."

Error: "Transport reported error: 1255: A network-related or instance-specific error occured while establishing a connection to SQL server." when attempting to perform a Check Connection on a SQL Server when using an Instance (Service Name)

When using the Authentication Type of "Password", Safeguard uses a TCP/IP connection to MS SQL. (TCP/IP is also used for Directory Account /Local System Accounts when "Use Named Pipe for Service Account connection" is unticked.)

When using a TCP/IP connection, Safeguard does not use the instance name in the ODBC connection string as the underlying ODBC driver ignores it. 
The Network Address and Port Number configured for the asset in Safeguard  determine the instance that Safeguard connects to. 

Ensure that TCP/IP is enabled on the SQL server and correct port is specified in Safeguard.

This can be validated from the SQL server. In SQL Server Configuration Manager, expand "SQL Server Network Configuration", select "Protocols for instancename". Ensure TCP/IP is enabled,  go to the "IP Addressess" tab and validate the "TCP Dynamic Ports" under IPAll. This port should be used in Safeguard. If any changes are made restart the SQL instance service.